1. Field of the Invention
This invention relates generally to patterned-media magnetic recording disks with patterned servo sectors, and to a method for making a master mold for nanoimprinting the patterned-media disks.
2. Description of the Related Art
Magnetic recording hard disk drives with patterned magnetic recording media have been proposed to increase data density. In patterned media, the magnetic recording layer on the disk is patterned into small isolated data islands arranged in concentric data tracks. To produce the required magnetic isolation of the patterned data islands, the magnetic moment of spaces between the islands must be destroyed or substantially reduced to render these spaces essentially nonmagnetic. In one type of patterned media, the data islands are elevated regions or pillars that extend above “trenches” and magnetic material covers both the pillars and the trenches, with the magnetic material in the trenches being rendered nonmagnetic, typically by “poisoning” with a material like silicon (Si). In another type of patterned media, the magnetic material is deposited first on a flat disk substrate. The magnetic data islands are then formed by milling, etching or ion-bombarding of the area surrounding the data islands.
Like conventional non-patterned or continuous-media disks, patterned-media disks are also required to have nondata servo regions that are used for read/write head positioning. The servo regions in the pre-etched type of patterned-media disks with elevated spaced-apart data pillars may also be patterned and thus contain elevated nondata servo islands or pillars that are separated by trenches. The servo pillars are typically “servowritten” or pre-magnetized during the manufacturing process and are not intended to be rewritten during normal operation of the disk drive. The proposed method for servowriting this type of disk is to DC “erase” the disk during manufacturing with a large magnet, leaving all of the servo pillars magnetized in the same direction. Thus for a patterned-media perpendicular magnetic recording disk, all of the servo pillars will have the same magnetization direction, i.e., either “into” or “out of” the surface of the disk. However, unlike the pattern of data pillars which are generally a periodic pattern of lines or dots, the servo pillars typically have different shapes and do not form a periodic pattern, but a generally arbitrary pattern. One such servo pattern is a chevron pattern of slanted or non-radial stripes that extend across the data tracks.
One proposed method for fabricating patterned-media disks is by nanoimprinting with a template or mold, sometimes also called a “stamper”, that has a topographic surface pattern. In this method the magnetic recording disk substrate with a polymer film on its surface is pressed against the mold. The polymer film receives the reverse image of the mold pattern and then becomes a mask for subsequent etching of the disk substrate to form the pillars on the disk. In one type of patterned media, the magnetic layer and other layers needed for the magnetic recording disk are then deposited onto the etched disk substrate and the tops of the pillars to form the patterned-media disk. In another type of patterned media, the magnetic layers and other layers needed for the magnetic recording disk are first deposited on the flat disk substrate. The polymer film used with nanoimprinting is then pressed on top of these layers. The polymer film receives the reverse image of the mold pattern and then becomes a mask for subsequent milling, etching or ion-bombarding the underlying layers. The mold may be a master mold for directly imprinting the disks. However, the more likely approach is to fabricate a master mold with a pattern of pillars corresponding to the pattern of pillars desired for the disks and to use this master mold to fabricate replica molds. The replica molds will thus have a pattern of recesses or holes corresponding to the pattern of pillars on the master mold. The replica molds are then used to directly imprint the disks. Nanoimprinting of patterned media is described by Bandic et al., “Patterned magnetic media: impact of nanoscale patterning on hard disk drives”, Solid State Technology S7+Suppl. S, September 2006; and by Terris et al., “TOPICAL REVIEW: Nanofabricated and self-assembled magnetic structures as data storage media”, J. Phys. D: Appl. Phys. 38 (2005) R199-R222.
The making of the master template or mold is a difficult and challenging process. The use of electron beam (e-beam) lithography using a Gaussian beam rotary-stage e-beam writer is viewed as a possible method to make a master mold capable of nanoimprinting patterned-media disks, but this method limits the areal bit density of patterned-media disks to about 500 Gbit/in2. Directed self-assembly of block copolymers has also been proposed for making the master mold and is believed capable of achieving areal bit densities of greater than 1 Terabit/in2. Pending application Ser. No. 12/141,062, filed Jun. 17, 2008 and assigned to the same assignee as this application, describes the use of directed self-assembly of block copolymers to make a master mold that allows the circumferential density of the subsequently nanoimprinted data pillars to be at least doubled from what could be achieved with a mold fabricated with just e-beam lithography. Pending application Ser. No. 12/604,333, filed Oct. 22, 2009 and assigned to the same assignee as this application, describes the use of directed self-assembly of two different block copolymers to make a master mold that has a chevron servo pattern. However, the interface regions between the circumferentially-spaced sets of stripes of the chevron pattern may exhibit defects due to the sharp transitions of the slanted stripes.
What is needed is a method using directed self-assembly of block copolymers to make a master mold for nanoimprinting patterned-media magnetic recording disks that have servo patterns without defective areas.